These modules and the "DART" tool they support have long been replaced
by CTest.

For policy-specific tests, use the version before the policy was
introduced.  Otherwise, use 3.5 where possible.

Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.

In commit 7a969fe21d (cmMakefile: Refactor API to better handle empty
config values, 2020-06-30, v3.19.0-rc1~567^2), calls to
`GetGeneratorConfigs` that pass `OnlyMultiConfig` only want to get any
configurations listed if the generator is multi-config.  Fix the
implementation to actually do that.

Fixes: #21316

Fixes: #21004

Use 3.3 or 2.8.12 where possible.

Fixes: #20172

Revert commit v3.8.0-rc1~305^2 (Remove CTestTestfile.cmake when BUILD_TESTING
is OFF, 2016-11-14) again.  We reverted it once in commit v3.8.0-rc3~22^2
(Revert "Remove CTestTestfile.cmake when BUILD_TESTING is OFF", 2017-03-06) but
it was accidentally restored by commit v3.11.0-rc1~387^2 (server: add
"ctestInfo" request to get test info, 2017-10-25), perhaps due to conflict
resolution during rebase.

We cannot remove `CTestTestfile.cmake` when testing is off because it breaks
projects that never enable testing but create their own `CTestTestfile.cmake`
manually instead.  Revert the change again and add a test case.